Path-integral study of positronium decay in xenon

We use path-integral Monte Carlo to study the properties of a quantum particle equilibrated in a classical Lennard-Jones fluid. By choosing 2m(e) for its mass, and potential parameters corresponding to xenon, we are able to model the behavior of thermalized positronium above the xenon critical temperature. We carefully study the local distortion of the fluid in the neighborhood of the quantum particle, and use this information to compute the annihilation rate as a function of density on two isotherms. The results compare favorably with experiment below the critical point density. Contrary to accepted views, we demonstrate that positronium remains in a self-trapped state at over twice the critical point density.